Mapping and monitoring of hydraulic fractures using vector magnetometers

ABSTRACT

A method for mapping and monitoring of hydraulic fracture includes capturing, using an array of sensors, a first magnetic image of a well pay zone. A second magnetic image is captured using the array of sensors, after a well bore is padded with a fluid. A background is established based on the first and the second magnetic images. A third magnetic image is captured using the array of sensors, after a doped proppant is injected into a stage. The third image is processed to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Patent Application 61/955,918, filed Mar. 20, 2014, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to process control, more particularly, to mapping and monitoring of hydraulic fractures using vector magnetometers.

BACKGROUND

In recent decades, an increasing trend used by the oil and gas industry for producing natural gas from unconventional resource plays has been the application of hydraulic fracturing, or fracking Examples of unconventional plays include tight sand formations containing natural gas, and shale reservoirs, such as the Marcellus shale formation located in the Northeastern United States.

Fracking involves the introduction of a fracking fluid at high pressure into a formation to cause stress fractures in the surrounding formation. This increases the permeability of the formation, thereby allowing more of the natural gas trapped within the formation to be collected. After fracturing, a proppant is introduced to keep open the resultant fractures. Fracking has proven to be revolutionary to the natural gas industry, and proponents of fracking believe it to be a major step for reducing dependence on foreign sources of natural gas. However, opponents claim that fracking fluid may contaminate ground water and often propose greater restrictions and better regulated procedures than those currently employed. The oil and gas industry has responded to concerns by attempting to map out the induced fracture network in conjunction with the use of “friendlier” fracking fluids. By mapping the induced fracture network, information may be obtained regarding the extension of the induced fractures and the proximity of the induced fractures to other underground features, such as underground faults. If induced fractures reach a natural fault that extends toward the surface, frac fluid may be able to migrate upward, approaching more shallow underground features such as the aquifer.

In an attempt to map induced fractures in typical fracking systems, micro-seismic arrays are arranged on the surface or placed down hole, and configured to listen for “pops” which occur when induced shear fractures intersect with existing natural fractures. Release of energy associated with the pops or seismic events travels via elastic wave propagation to an array of geophones (e.g., receivers) which are used to triangulate the location of the event or hypocenter. The seismic data is collected and triangulation calculations are performed to locate the source of the seismic activity. However, seismic sensors suffer from reduced sensitivity due to very weak signals and the attenuation of seismic signals by natural fractures. Additionally, there is a high degree of uncertainty in the measurement of micro-seismic activity due to the inability to know a priori when a fracture event is going to occur. The seismic monitoring approach is also known as passive micro-seismic monitoring, in which the term passive refers to the lack of controlled energy input resulting from the use of explosives, air guns, or thumpers for reflection seismic surveys. Because there is no controlled energy input, the event time for a shear pop is unknown. Thus, there are more unknowns to solve for and the mapping problem is more difficult. Also, there may be overlapping pops, which complicate the process of sorting out the seismic return signals (e.g. first arrivals).

Another approach involves the placement of very sensitive tilt meters which act like a carpenter's level to measure movement of the earth's surface due to expansion and shifting of subsurface formations due to the introduction of a pressurized fluid and the resulting induced fracture network.

Still another approach has been suggested in which radioactive isotopes are added to the fracking fluid and subsequently monitored using gamma ray spectrometry. However, this approach introduces additional potentially dangerous substances into the ground, where the trend is toward less invasive materials. These known methods fail to relate any information regarding proppant injected into induced fractures during hydraulic fracturing. For example, rock may be fractured under pressure, but the fractures may close once the source of force against the fracture is removed or reduced. Therefore, even if the known methods perform as intended, a true picture of the induced fracture network is not obtained. Alternative methods and systems for mapping induced hydraulic fractures are desired.

SUMMARY

In some aspects, a method for mapping and monitoring of hydraulic fracture includes capturing, using an array of sensors, a first magnetic image of a well pay zone. A second magnetic image is captured using the array of sensors, after a well bore is padded with a fluid. A background is established based on the first and the second magnetic images. A third magnetic image is captured using the array of sensors, after a doped proppant is injected into a stage. The third image is processed to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage.

In another aspect, a system for mapping and monitoring of hydraulic fracture includes an array of sensors and a processor. The array of sensors is configured to capture magnetic images including a first magnetic image of a well pay zone, and a second magnetic image including a magnetic image captured after a well bore is padded with a fluid. The processor is configured to provide a background image based on the first and the second magnetic images. A third magnetic image is captured by the array of sensors after a doped proppant is injected into a stage. The processor is configured to process the third magnetic image to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage.

In yet another aspect, a method includes capturing a first image of a well pay zone, capturing a second image after a well bore is padded with a fluid, providing a background image based on the first and the second images, capturing a third image after a doped proppant is injected into a stage, and processing the third image to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage. The first, second, and third images comprise magnetic images.

The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows can be better understood. Additional features and advantages of the disclosure will be described hereinafter, which form the subject of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein:

FIGS. 1A-1B are diagrams illustrating examples of a high-level architecture of a system for mapping and monitoring of hydraulic fracture and an environment where the system operates, according to certain embodiments;

FIG. 2 is a high-level diagram illustrating an example of implementation of hydraulic fracturing of a well to release gas reserves, according to certain embodiments;

FIG. 3A is a diagram illustrating an example background magnetic signature of a well, according to certain embodiments;

FIG. 3B is a diagram illustrating an example implementation of a mapping system for hydraulic fracturing of the well shown in FIG. 3A, according to certain embodiments;

FIG. 4 is a diagram illustrating an example of a method for mapping and monitoring of hydraulic fracture, according to certain embodiments;

FIG. 5 is a diagram illustrating an example of a system for implementing some aspects of the subject technology; and

FIG. 6 is a diagram illustrating examples of primary and secondary magnetic fields in the presence of a doped proppant, according to certain embodiments.

DETAILED DESCRIPTION

The detailed description set forth below is intended as a description of various configurations of the subject technology and is not intended to represent the only configurations in which the subject technology may be practiced. The appended drawings are incorporated herein and constitute a part of the detailed description. The detailed description includes specific details for the purpose of providing a thorough understanding of the subject technology. However, it will be clear and apparent to those skilled in the art that the subject technology is not limited to the specific details set forth herein and may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the subject technology.

The present disclosure is directed, in part, to mapping and monitoring of hydraulic fractures using vector magnetometers. Magnetic images are capture at various phases of the hydraulic fracturing operation (also referred to as “fracking”), which include padding and injection of fracking (frac) fluid and proppant, as described in more detail herein. The subject technology allows monitoring and adjustment of the fracking operation by providing a map of the distribution of the frac fluid and proppant in various stages.

The disclosed solution can be used in conjunction with micro-seismic monitoring. Micro-seismic monitoring is very challenging due to the fact that initial times for the shear fracture events are unknown, which results in large uncertainty in the depth migration problem of seismic processing. Other limiting factors include observation of only shear fractures, and the fact that fracture events themselves don't indicate whether or not the induced fracture was effectively propped open subsequent to removal of pressurized frac fluid.

The subject solution provides indication of proppant penetration into the fracture network during and subsequent to the frac process, which is the key to better controlling the overall fracking process. Fracking is typically a multiple stage or zonal process per each well. The disclosed solution also enables adapting initial frac plan to evolving conditions.

FIGS. 1A-1B are diagrams illustrating examples of a high-level architecture of a system 100A for mapping and monitoring of hydraulic fracture and an environment 100B where the system operates, according to certain embodiments. The system 100A includes a sensor array 102 including multiple sensors 103, an analyzer 105, and an output device 109. Each sensor 103 includes at least a magnetometer communicatively coupled to the analyzer 105. The analyzer 105 includes one or more processors 106, memory 108 and an interface 104. Each sensor may communicate data signal to the analyzer 105. The communication between the sensors and the analyzer 105 may be wired, optical, or wireless communication. The analyzer 105 may communicate with the sensors 103 individually or with the sensor array 102 through the interface 104 to receive sensor data. The analyzer 105 may store the sensor data received from the sensors 103 or the sensor array 102 to the memory 108. The stored data may be accessed by processor(s) 106 for processing the data subsequent to the sensors storing their respective data. The processor(s) 106 may be configured to receive executable instructions for processing the data according to the constrained geophysical processing described herein. The signals produced by the sensor array 102 may include magnetic imaging data for generation of a magnetic profile of a region defined by the well which is intended to be processed using hydraulic fracturing. Each magnetometer sensor 103 may save its vector field measurement every few minutes throughout the entire fracking process. All saved data is time tagged by some simple means such as a common clock or a trigger for later processing of the data.

The memory 108 is in communication with processor 106 and the interface 104. Memory 108 may store information, such as the sensor array 102 signals received by the analyzer 105. Further, memory 108 may store magnetic images or signals that have been received from sensor array 102 and further processed by processor(s) 106. The interface 104 communicates data from the analyzer 105 to an output device 109. The output device 109 may be any device or apparatus that can communicate information about the processed signals received from sensor array 102. For example, the output device 109 may be a display configured to display a graphical depiction of a well site, including a mapping of an induced fracture network produced during hydraulic fracturing. In some aspects, the output 517 may be a printing device providing information (e.g. reports) relating to a hydraulic fracturing operation.

In one or more implementations, the sensors 103 are arranged in a sensor array 102 and communicatively connected to analyzer 105. The sensors 103 may include a magnetometer for measuring a magnetic field in the proximity of the sensor 103, which is communicated to the analyzer 105. The magnetic fields measured by sensor array 102 may be related to a well being processed using hydraulic fracturing. The magnetic field measured by the sensors 103 may include magnetic fluences relating to the Earth's magnetic field, as well as remnant magnetism in the rock formation and magnetic properties of the well apparatus itself, such as the well casing. As the well is fractured by injecting fluid and proppants into the well bore at selected stages along the bore, the magnetic field in the region of the hydraulic fluids and proppants affect the surrounding magnetic fields that are subsequently measured by the sensors 103. As hydraulic fracturing proceeds in the well, subsequent magnetic images are captured by the sensor array 102 and communicated to the analyzer 105. The received magnetic images are processed by processor(s) 106 to determine changes in the magnetic profile between successive magnetic images captured by the sensor array 102. The changes are processed to map the distribution of frac fluid and proppant in the well, which are indicative of the induce fracture network into which the fluid and proppant has flowed during hydraulic fracturing.

FIG. 1B shows the environment 100B, which is representation of the geology of natural gas resources. The growth of natural gas reserves and production from shale formations has sparked interest in the nation's natural gas resources. The diagram in FIG. 1B shows the geologic nature of most major sources of natural gas in the United States in schematic form. Gas rich shale 110 is the source rock for many natural gas resources, but until recently, has not been a focus for production. Horizontal drilling and hydraulic fracturing have made shale gas an economically viable alternative to conventional gas resources. Conventional gas accumulations 140, 150, or plays, occur when gas migrates from gas-rich shale into an overlying sandstone formation, and then becomes trapped by an overlying impermeable formation, called the seal 130. Associated gas 140 accumulates in conjunction with oil 120, while non-associated gas 150 does not accumulate with oil. Tight sand gas accumulations 160 occur when gas migrates from a source rock into a sandstone formation 170, but is limited in its ability to migrate upward due to reduced permeability in the sandstone. Finally, coal bed methane 180 does not originate from shale, but is generated during the transformation of organic material to coal.

Conventional gas accumulations 140, 150 may be accessed via horizontal drilling techniques in which the well bore is substantially vertical. To access non-conventional plays such as gas-rich shale formations 110, horizontal drilling techniques in which the well bore 195 extends substantially horizontally 196 may be needed. Generally, the permeability of unconventional reservoirs is too low for production, thus requiring directional drilling and well stimulation. For example, the permeability of a typical shale formation may be on the order of 10⁻⁹ Darcy. Tight sand formations may have permeability of about 10⁻⁶ Darcy. In contrast, a conventional play may have permeability of 10⁻² Darcy.

FIG. 2 is a high-level diagram illustrating an example of implementation of hydraulic fracturing of a well to release gas reserves, according to certain embodiments. A well head 201 is installed at ground level and attached to a water supply from a storage container 203 via a pump 205. The pump provides a frac fluid at a sufficient pressure in the well bore 295 to produce fracturing of the underlying shale layer 210. Natural gas trapped within the natural fissures 220 in the shale layer 210 are released as the newly formed fractures expand existing fissures while creating newly induced fractures and pathways through the remaining shale formation 210.

Shale is a finely grained sedimentary form of rock. Spaces between the grains are typically very small. As natural gas is formed, some of the gas becomes trapped within these small spaces. Using conventional mining and drilling techniques these trapped resources are difficult to access. Despite the resource richness of these sources, the production from wells in these types of formations has proven to be economically infeasible. Yet despite the inability to access the trapped gas due to the high impermeability of the shale, the shale contains a high volume of pore space that may contain substantial amounts of gas collected over long geological timeframes. Hydraulic fracturing provides access to this pore space, allowing the trapped gas 230 migrate toward the well bore 295 and be collected at the well head 201.

Frac fluid is stored near the well head 201 in storage container 203. The frac fluid is provided to the well bore 295 under pressure provided by the pump 205. The frac fluid is primarily water, but other additives or chemicals may be added to the frac fluid. For example, water pumped into the shale layer 210 at pressure, creates new fractures in the grains of the shale formation. When the pressure is relieved, such as by turning off the pump 205, the newly formed cracks in the shale tend to reclose under the pressure caused by the mass of the overlying layers. To maintain the openings created by the hydraulic pressure, a substance called a proppant 240 is added to the frac fluid. The proppant 240 props open the newly formed cracks 220 to allow the trapped natural gas 230 to migrate toward the well bore 295. The proppant 240 typically includes sand, which has a compressibility sufficient to maintain the openings in the shale, while providing enough permeability to allow the migration of the natural gas within the shale formation. While frac sand is a commonly used proppant, other materials, for example, aluminum beads, ceramic beads, sintered bauxite and other materials may be used, provided the material is crush-resistant and provides adequate permeability.

Other materials or chemicals may be added to frac fluid to provide additional functionality. For example, thickening agents may be added to the frac fluid to form a gel, which is effective at carrying the proppant particles deep into the rock formation. Other chemicals may be added to reduce friction, maintain rock debris from the fracking process in suspension for ease of removal, prevent corrosion of equipment, kill bacteria, control pH, as well as perform other functions.

The frac fluid is introduced to the well bore 295 under pressure (as indicated by arrow 270) and enters the natural fissures 220 located within the shale layer 210. Hydrostatic pressure builds in the shale until the pressure creates force which exceeds the tensile strength of the shale grains causing the grains to fracture and split. The entire well bore 295 does not need to be pressurized. Plugs may be placed beyond the regions of shale being targeted for fracturing to produce the desired pressure within a targeted region or stage.

The well bore 295 may extend from the surface for thousands of feet to reach the shale layer 210 below. Overlying layers, include the aquifer 250 which may provide the water supply for the area surrounding the well 200. To protect the water supply from contamination, the well bore 295 is lined with a steel casing 260. The space between the outside of the steel casing 260 and the walls of the well bore 295 are then filled with concrete to a depth greater than the aquifer 250. As the well bore 295 approaches the depth containing the gas-rich shale formation 210, the well bore 295 is angled to a horizontal or nearly horizontal direction to run longitudinally through the shale formation 210. As the pressurized frac fluid is applied to the shale layer 210 the existing fissures 220 are expanded and newly formed fractures are created. As shown in detail in the inset of FIG. 2, the frac fluid and proppant 240 enter the existing fissures 220 and create new fissures. Proppant particles 240 contained in the frac fluid hold the fissures open and provide permeability for gas 230 located within the fissures to migrate through the frac fluid and proppant particles to the well bore 295 and back to the surface.

During production of a non-conventional play, a horizontal pay zone extending about 4,000 feet through the pay zone may be established. Fracturing is performed along the horizontal pay zone in typically uniform stages extending about 400 feet. For a typical fractured well, 10-20 million square feet of additional surface area is created by the fractures. The fracking is performed beginning at the toe or end of the well, and processed stage by stage back toward the well opening. Fracking a typical well requires about 2.5 million pounds of proppant and about 4-6 million gallons of frac fluid. The fracturing process seeks to push proppant radially out from the well bore into the formation up to 1,000 feet. Ideally, fractures create sheet-like openings that extend orthogonally to the direction of the well bore. To this end, wells are typically drilled based on prior knowledge of the in situ stress state of the rock formation. Spacing for the fracturing stages are selected based, at least in part, on the anticipated induced fracture and empirically determined flow rates into the fracture network to ensure that production is commensurate with the intended 20-30 year life expectancy of a typical well installation. A production field may contain a number of wells configured as described above. The wells are spaced according to the corresponding designed pay zone of each well. The use of hydraulic fracturing is intended to maximize the stimulated rock volume (SRV) per dollar cost of production.

Experience has shown, however, that induced fractures define complicated networks of fractures rather than the ideal sheet-like openings. Accordingly, mapping the occurrence and location of actual fractures becomes valuable in determining the effectiveness of the current operations, and provides insight into future actions to maximize production efficiency of the well. Factors that create uncertainty in the hydraulic fracturing process include the loss of frac fluid and proppants to pre-existing or natural fractures which may open further during the fracking process. Injected fluid and proppant is accommodated, (e.g., space/volume become available) by the compliance of the surrounding rock which becomes compressed, and thereby alters the rock's stress state. This changes the stress field from one stage's fracture to the next. This results in added uncertainty as to the final placement of proppants to maintain openings formed by the fracking after the hydraulic pressure is removed.

Mapping induced fractures caused by hydraulic fracturing allows for greater production and maximized stimulated reservoir volume (SRV). In addition, concerns expressed over the process of fracking, including the proliferation of the fracking materials into the environment, may require accurate mapping of induced fractures and the introduction of frac fluids and proppants to those fractures to meet further regulatory requirements designed to control and regulate impact to the environment caused by hydraulic fracturing.

Presently, attempts at mapping fractures include passive micro-seismic monitoring. In micro-seismic monitoring, a passive array of seismic sensors is arranged at the surface overlying the fractured pay zone, or the sensors may be placed down hole in the fracked well or in a nearby observation well. The seismic sensors are configured to detect shear pops that occur when an induced tensile crack intersects with a natural fracture which emits a popping type of impulse. The impulses are converted to signals which are processed to determine the source of the impulse. Micro-seismic monitoring is passive. That is, no active seismic signal is generated and used to create returned signals. The sensors merely monitor the surroundings for seismic activity if and when such activity occurs. Since it not known when a fracture may be induced by the hydraulic pressure, or where such fractures may occur, there is considerable uncertainty in seismic monitoring. This uncertainty is compounded by the very low energy seismic signals which must be detected. Further, seismic monitoring does not provide insight as to the effective placement of proppants, as the impulses used to generate signals occur at the initiation of an induced fracture and do not indicate if the fractures were successfully propped open, or reclosed after the initial fracture. Therefore, it is difficult to verify that the mapping information generated is reliable. The subject solution may be used alone or in cooperation with existing techniques including micro-seismic monitoring.

According to one or more implementations, an array of sensors is placed on or near the surface of a well or active pay zone. The array of sensors includes at least a magnetometer sensor for measuring a magnetic field around the sensor. In an alternative embodiment, one or more of the magnetometer sensors may be placed down hole in the well, although this is not a requirement and a system may be embodied using solely surface arrays. The environment around the well has a magnetic signature that may be measured by the sensor array. For example, the Earth's magnetic field will influence the overall magnetic signature in the area of the well. Additionally, remnant sources of magnetic fields, such as the host rock or intrusions of magnetite further influence the magnetic field sensed by the array of magnetometer sensors. Further, as the well casing is driven down in the well bore, the well casing tends to become magnetized, thereby affecting the magnetic field measured at the magnetometer sensor array.

According to an embodiment, a process includes placing an array of sensors proximate to a well pay zone. Prior to introducing any frac fluid for hydraulic fracturing, a baseline magnetic profile is established by measuring the magnetic signature prior to any hydraulic fracturing being performed. The baseline magnetic signature includes the Earth's magnetic field, remnant sources of magnetism in the earth and the magnetic field which is associated with the well casing. The magnetometer sensor may be based on a diamond nitrogen vacancy (DNV) sensor. A DNV sensor includes a synthetic diamond substrate which is created having intentional impurities introduced into the carbon lattice structure of the diamond. Nitrogen atoms replace the carbon atoms at varying locations in the lattice, thereby creating vacancies which contain electrons. The electrons have various spin states which may be measured. The spin states are sensitive to the surrounding magnetic environment. As the magnetic environment changes, the spin states of the electrons change and the difference in spin may be correlated to the corresponding change in the magnetic environment. Magnetometers based on DNV technologies are very sensitive and can detect small changes in magnetic fields in a sensor which is considerably smaller than other technologies. For example, a typical conventional magnetometer capable of detecting small changes in the magnetic profile of a well's pay zone may require a sensor which is the size of a small van. In contrast, a DNV based magnetometer may be embodied in a sensor the size of a cellular telephone or smaller. Thus, a number of small, very sensitive magnetometers can be carried on site and arranged in an array about the surface in the area defining the well pay zone.

FIG. 3A is a diagram illustrating an example background magnetic signature 300A of a well, according to certain embodiments. A well may include a bore 320 that is drilled vertically from the surface to a desired depth, at which point the bore 320 is extended horizontally along the pay zone. A well casing 325 is inserted into the bore to insulate the well bore 320 from the surrounding rock formation and to prevent introduction of mining materials into the surrounding rock near the surface. As the well casing 325 is driven into the rock formation, the casing tends to become magnetized and form the magnetic field 326. The surrounding rock formation contains naturally occurring remnant magnetism 316 which may be in the host rock or intrusions of other materials such as magnetite 315. In addition, the Earth has its own global magnetic field 301 that extends through the area defined by the well and its pay zone.

FIG. 3B is a diagram illustrating an example implementation of a mapping system 300B for hydraulic fracturing of the well shown in FIG. 3A, according to certain embodiments. The mapping system 300B includes the sensor array 311 including magnetometer sensors 310 arranged on the surface in an area defining the pay zone of the well. According to some aspects, a one-to-one placement of magnetometers with geophones (e.g., for concurrent micro-seismic mapping) at the surface may be used. This configuration provides a wide aperture and allows for triangulating locations. The addition of magnetometer data requires minimal modification to procedures already established for micro-seismic techniques. Where the well is cased, monitoring the opened holes may involve introducing sensors at a subsurface level. Downhole placements of sensors may also be used to provide much stronger signals.

The sum of the magnetic fields created by the Earth's magnetic field 301, the remnant magnetism in the host rock 315, and additional magnetic influence of the mining materials, such as the well casing 326, define a baseline magnetic field of the well region which is measured by the array of magnetometers at the surface before any introduction of fracking material into the well bore 320. Frac fluid is introduced at high pressure to the well bore opening and the well bore 320 is filled with the fluid through the bore 320 to the toe of the well which initiates fractures in the rock. The fluid introduced prior to introducing proppant and other additives to the fluid is called padding. A typical well may receive millions of gallons of frac fluid in addition to millions of pounds of proppant 330. This large additional mass is received by the surrounding formation and may affect the surrounding magnetic signature. For this reason, the sensor array 311 may be configured to measure the baseline magnetic signature of the well adjusted for the additional mass provided by the padding fluid and proppant 330.

After the baseline magnetic signature has been measured, introduction of additional frac fluid and proppant 330 to the well may begin. The fluid is provided to the well in stages. A typical 4,000 foot horizontal pay zone may be hydraulically fractured in stages of about 400 feet at a time. In some aspects, the first stage is the length of the well bore 320 closest to the toe. Subsequent stages are processed sequentially, working from the toe back to the well opening. As the frac fluid is introduced to a new stage, the sensor array 311 measures the magnetic signature of the well pay zone region. The addition of the fluid causes hydraulic fracturing of the rock 305 surrounding the horizontal well bore in the area of the stage presently being processed. Changes from the baseline measured magnetic signature indicate the presence of the additional fluid and proppant 330 as it extends into the new induced fractures caused by the pressurized fluid. The changes may be monitored as subsequent stages are processed, with incremental changes in the measured magnetic signature being analyzed to provide insight into the progress and location of the newly formed fracture network.

To augment the information received at the sensor array as each stage is processed, the frac fluid and/or the proppant 330 may be treated or infused with a magnetically susceptible material. For example, small ferrite particles may be added to the proppant particles 330. The ferrite particles have a greater effect on the overall magnetic signature of the area to which they are introduced due to their magnetic susceptibility. According to some implementations, the proppant 330 is mixed with a magnetically susceptible material. In other implementations, the frac fluid may be mixed with the magnetically susceptible material, or both the fluid and the proppant 330 may be treated with the magnetically susceptible material. The differential magnetic signature is determined based on measuring the magnetic signature with the magnetometer sensor array after the magnetically susceptible proppant or fluid is added to a processing stage, and compared with the previous measured magnetic signatures measured prior to the addition of the proppant or fluid.

When adding a magnetic susceptible material to the frac fluid or the proppant 330, the material is selected such that the addition of the material does not substantially increase the weight of the proppant of fluid. Along the horizontal pay zone, fractures in the rock extend in varying directions in a web-like manner radially from the horizontal well bore. Therefore, as the well is hydraulically fractured, the frac fluid and proppant 330 must flow from the well bore in all radial directions, including upward against the force of gravity. If the added magnetically susceptible material adds too much weight to the fluid or the proppant 330, the heavier material will tend to settle due to gravity and not flow into the upward regions of the surrounding rock formation.

A sequence of changes in the passive magnetic images captured by the magnetometer sensors during the fracking process are used to determine the proppant placement. The frac fluid and/or the synthetic proppant may be doped with a magnetically susceptible material. Monitoring of the hydraulic fracturing process continues as multiple magnetic images are captured throughout the proppant injection phase. The background or baseline magnetic profile is removed from the images formed throughout the propping phase. Constrained geophysical processing of the resulting group of magnetic images provides information about the distributions of fluid and proppant.

FIG. 4 is a diagram illustrating an example of a method 400 for mapping and monitoring of hydraulic fracture, according to certain embodiments. According to the method 400, using an array of sensors (e.g., 102 of FIG. 1A or 311 of FIG. 3A), a first magnetic image of a well pay zone (e.g., 200 of FIG. 2) is captured (410). Using the array of sensors, a second magnetic image is captured after a well bore (e.g., 295 of FIG. 2) is padded with a fluid (420). A background is established based on the first and the second magnetic images (430). Using the array of sensors, a third magnetic image is captured after a doped proppant (e.g., 240 of FIG. 2) is injected into a stage (e.g., 220 of FIG. 2) (440). The third image is processed to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage (450).

FIG. 5 is a diagram illustrating an example of a system 500 for implementing some aspects of the subject technology. The system 500, for example, may be a system that the analyzer 105 of FIG. 1A is implemented on or may perform the functionalities of the analyzer 105 of FIG. 1A. In some implementations, the system 500 may perform simulations described herein. The system 500 includes a processing system 502, which may include one or more processors or one or more processing systems. A processor can be one or more processors. The processing system 502 may include a general-purpose processor or a specific-purpose processor for executing instructions and may further include a machine-readable medium 519, such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in a machine-readable medium 510 and/or 519, may be executed by the processing system 502 to control and manage access to the various networks, as well as provide other communication and processing functions. The instructions may also include instructions executed by the processing system 502 for various user interface devices, such as a display 512 and a keypad 514. The processing system 502 may include an input port 522 and an output port 524. Each of the input port 522 and the output port 524 may include one or more ports. The input port 522 and the output port 524 may be the same port (e.g., a bi-directional port) or may be different ports.

The processing system 502 may be implemented using software, hardware, or a combination of both. By way of example, the processing system 502 may be implemented with one or more processors. A processor may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device that can perform calculations or other manipulations of information.

In one or more implementations, the transformation means (e.g., algorithms) and the signal processing of the subject technology may be performed by the processing system 502. For example, the processing system 502 may perform the functionality of the processor 106 of FIG. 1A or other or computational functions or simulations described herein.

A machine-readable medium can be one or more machine-readable media. Software shall be construed broadly to mean instructions, data, or any combination thereof, whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. Instructions may include code (e.g., in source code format, binary code format, executable code format, or any other suitable format of code).

Machine-readable media (e.g., 519) may include storage integrated into a processing system such as might be the case with an ASIC. Machine-readable media (e.g., 510) may also include storage external to a processing system, such as a Random Access Memory (RAM), a flash memory, a Read Only Memory (ROM), a Programmable Read-Only Memory (PROM), an Erasable PROM (EPROM), registers, a hard disk, a removable disk, a CD-ROM, a DVD, or any other suitable storage device. Those skilled in the art recognizes how best to implement the described functionality for the processing system 502. According to one aspect of the disclosure, a machine-readable medium is a computer-readable medium encoded or stored with instructions and is a computing element, which defines structural and functional interrelationships between the instructions and the rest of the system, which permit the instructions' functionality to be realized. Instructions may be executable, for example, by the processing system 502 or one or more processors. Instructions can be, for example, a computer program including code for performing methods of the subject technology.

A network interface 516 may be any type of interface to a network (e.g., an Internet network interface), and may reside between any of the components shown in FIG. 5 and coupled to the processor via the bus 504.

A device interface 518 may be any type of interface to a device and may reside between any of the components shown in FIG. 5. A device interface 518 may, for example, be an interface to an external device that plugs into a port (e.g., USB port) of the system 500.

FIG. 6 is a diagram illustrating examples of primary and secondary magnetic fields in the presence of a doped proppant, according to certain embodiments. According to an aspect of the disclosure, FIG. 6 depicts a scenario wherein proppant doped with magnetically susceptible matter 603 (e.g. the dopant) becomes magnetized and aligns with an external magnetic field, H ₀ 601. Such external magnetic field may consist of the Earth's natural (geomagnetic) field, as well as possibly that of the surrounding rocks having remnant magnetization, and a magnetized well casing. The external field 601 is commonly/synonymously referred to as the primary, background, or inducing field, which may be represented as a vector quantity having strength or magnitude, and direction.

Magnetization is also represented as a vector quantity, and the magnetization of the volume of doped proppant 603 depicted below is labeled M. Upon becoming magnetized, the susceptible proppant 603 gives rise to an induced or secondary field 605, H _(s). The induced field 605 is distinct from, but caused, by the primary field 601. The total magnetic field is then determined as the superposition of the primary field 601 and secondary field 605. In the simplest case (e.g. isotropic), magnetization relates to the total field by a scalar-valued susceptibility χ, according to:

M=χ H =χ( H ₀ + H _(s))  Eq. (1)

In a non-limiting embodiment, a standard approximation may be made which assumes the primary field 601 is significantly greater than the secondary field 605. Thus, the system's calculation may be made according to M≠χ H ₀ and wherein the magnetization is parallel to the primary field 601 and is linearly proportional to it through the susceptibility at any given location.

Generally, the vector field at an observation or measurement point P due to a distribution of magnetized matter (e.g. doped proppant) within a source region Ω is given by:

$\begin{matrix} \begin{matrix} {{\overset{\rightarrow}{H}(P)} = {{{\overset{\rightarrow}{H}}_{0}(P)} + {{\overset{\rightarrow}{H}}_{s}(P)}}} \\ {= {{{\overset{\rightarrow}{H}}_{0}(P)} + {\frac{1}{4\; \pi}\underset{\Omega}{\int{\int\int}}{{\overset{\rightarrow}{M}(\xi)} \cdot {\bigtriangledown\bigtriangledown}}\frac{1}{\rho \left( {P,\xi} \right)}{\Omega}}}} \end{matrix} & {{Eq}.\mspace{14mu} (2)} \end{matrix}$

Given the quantities as previously defined, and ξ taking on all locations within the relevant source magnetic region. However, using the standard approximation this reduces to a model for the secondary field 605 depending on the susceptibility distributed throughout the relevant (i.e., non-negligible magnetic source) domain:

$\begin{matrix} {{{\overset{\rightarrow}{H}}_{s}(P)} = {\frac{1}{4\; \pi}\underset{\Omega}{\int{\int\int}}{(\xi)}{{{\overset{\rightarrow}{H}}_{0}(\xi)} \cdot {\bigtriangledown\bigtriangledown}}\frac{1}{\rho \left( {P,\xi} \right)}{\Omega}}} & {{Eq}.\mspace{14mu} (3)} \end{matrix}$

The magnetic source domain for an embodiment of the disclosure comprises the subsurface region surrounding the well that is being fracked, and extending outward from the well to a distance greater than the proppant would reasonably be expected to reach.

If the primary field 601 existing prior to injecting any doped proppant or frac fluid is complicated by unknown but significant remnants, then Eq. (2) may be used and the magnetization vector may be solved. Alternatively, Eq. (3) may be used to solve for the scalar susceptibility distribution assuming the primary field vector is known throughout the domain of interest, which is taken to be Earth's geomagnetic background, and is well characterized. This approach may represent a simpler implementation.

Consistent with the assumptions stated above, the difference between DNV-based vector magnetic field measurements taken before and during the injection of doped proppant comprises a measure of the secondary field 605 modeled by Eq. (3) above, induced throughout the fracking process.

According to an aspect of the subject solution, the subsurface domain Ω surrounding the well is subdivided into many model “cells” that are right rectangular prisms of uniform size (other geometric shapes can be used but it is much less common). The unknown susceptibility of the material region associated with each model cell is taken to be constant. Cell sizes are chosen so that this approximation is reasonable, while also being large enough to keep the overall problem tractable (e.g. not too many cells), yet small enough to offer useful resolution (e.g. smooth variation) of the susceptibility being solved for.

After this discretization of the domain into many smaller discrete, uniform subdomain “cells,” the susceptibilities for each cell being held constant can be removed from the volume integral and the integrals evaluated and arranged in a coefficient matrix (G) which multiplies the unknown susceptibilities (m) of each cell to compute secondary field values (d) that are expected to match the measured values. This forward model comprises a simple matrix-vector multiplication stated as:

d=Gm  Eq. (4)

The influence coefficient (G) maps the susceptibility values of all cells in the modeled domain to magnetic field values at each measurement point. As there are many more cells in the model than there are measurement locations, this problem is severely underdetermined and has no unique solution (e.g. it has an infinite number of solutions). This is typical of geophysical inversion problems.

Regularized inversion provides a solution to this dilemma and is a mainstay of geophysics, wherein additional constraints are introduced to yield uniqueness and enable solving for the many unknowns. Types of constraints vary widely, ranging from totally artificial and mathematically contrived, to constraints that are very much physics-based and well applied to certain problems.

A general formulation that encapsulates most of these approaches comprises the simultaneous minimization of data misfit and constraint violation. Data misfit is the difference between measured data and modeled data reconstructed by the forward model of Equation (4) for a specified set of cell susceptibilities. This can be written as a scalar, two-term performance index or cost function:

φ(m)=φ_(d)(m)+γφ_(m)(m)  Eq. (5)

where φ_(d) represents the data misfit term that takes on large values when a specified set of susceptibilities poorly reconstructs (via the forward model of Eq. (4)) the measured magnetic field values, and small values when the data is well matched. A quadratic form is common:

φ_(d)(m)=({tilde over (d)}−d)^(T) R ⁻¹({tilde over (d)}−d)  Eq. (6)

where the tilde (˜) annotation indicates actual measured data and square matrix (R) is the measurement error covariance matrix associated with the data. Accordingly, individual data entries known to be very accurate may require being very closely matched by the reconstruction. Otherwise their mismatch produces large penalties.

The term φ_(m) is a model adjustment term that embodies problem constraints that give uniqueness to the problem while also providing physical insight to the problem being solved. A simple example for this term is one that takes on large values for specified susceptibilities that differ greatly from a-priori values (note the a-priori values are often zero, which for a hydraulic fracturing application implies no proppant is pushed into the geologic subdomain corresponding to a cell of the forward model). A simple quadratic form for this term is:

φ_(m)(m)=(m ₀ −m)^(T) W(m ₀ −m)  Eq. (7)

where m₀ comprises the a-priori susceptibilities of the cells one intends to keep the solution near, and the square matrix (W) reflects the possibly differential importance or preference of keeping certain cell values closer to their a-priori values than others. The non-diagonal entries of W may be represented as zero entries, wherein W is diagonal and hence symmetric. Diagonal entries of W are all positive-valued.

Returning to the overall performance index of Eq. (5), the second (model adjustment) term is weighted by a scalar (γ) to achieve a balance between the two terms. For example, (γ) is typically heuristically adjusted so the overall performance index is evenly apportioned between the data misfit and model adjustment terms.

Susceptibilities are then solved for the quadratic case as:

m=(G ^(T) R ⁻¹ G+γW)⁻¹(G ^(T) R ⁻¹ {tilde over (d)}+γWm ₀)  Eq. (8)

The above described solutions provide the benefit of being easy to solve. The model adjustment term may encapsulate the following constraints, which may be particularly useful for embodiments according to this specification: (1) The well geometry is known a-priori, so model cells outside the fracked stage and potentially its neighboring stages are unlikely to have significant changes in their susceptibility; (2) the total amount of susceptible matter pumped down the well is known and must be matched by the recovered model; (3) alternatively to the quadratic adjustment term of Eq. (7) allowing small adjustment of all susceptibilities, a so-called focused inversion may be implemented wherein only susceptibilities of a subset (e.g. minimum) number of model cells are allowed to change during the solution.

The geophysical inversion calculations may be implemented in hardware, software or a combination of hardware and software, for example by the processing system 502 of FIG. 5. A general purpose computer processor (e.g., 502 or processor 106 of FIG. 1A) for receiving magnetic and/or micro-seismic signals may be configured to receive and execute computer readable instructions. The instructions may be stored on a computer readable medium in communication with the processor. One or more processors may be used for calculation some or all of the magnetic and/or micro-seismic signals according to a non-limiting embodiment of the present disclosure.

The foregoing description is provided to enable a person skilled in the art to practice the various configurations described herein. While the subject technology has been particularly described with reference to the various figures and configurations, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the subject technology.

One or more of the above-described features and applications may be implemented as software processes that are specified as a set of instructions recorded on a computer readable storage medium (alternatively referred to as computer-readable media, machine-readable media, or machine-readable storage media). When these instructions are executed by one or more processing unit(s) (e.g., one or more processors, cores of processors, or other processing units), they cause the processing unit(s) to perform the actions indicated in the instructions. In one or more implementations, the computer readable media does not include carrier waves and electronic signals passing wirelessly or over wired connections, or any other ephemeral signals. For example, the computer readable media may be entirely restricted to tangible, physical objects that store information in a form that is readable by a computer. In one or more implementations, the computer readable media is non-transitory computer readable media, computer readable storage media, or non-transitory computer readable storage media.

In one or more implementations, a computer program product (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

While the above discussion primarily refers to microprocessor or multi-core processors that execute software, one or more implementations are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In one or more implementations, such integrated circuits execute instructions that are stored on the circuit itself.

Although the invention has been described with reference to the disclosed embodiments, one having ordinary skill in the art will readily appreciate that these embodiments are only illustrative of the invention. It should be understood that various modifications can be made without departing from the spirit of the invention. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and operations. All numbers and ranges disclosed above can vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any subrange falling within the broader range are specifically disclosed. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted. 

What is claimed is the following:
 1. A method for mapping and monitoring of hydraulic fracture, the method comprising: capturing, using an array of sensors, a first magnetic image of a well pay zone; capturing, using the array of sensors, a second magnetic image after a well bore is padded with a fluid; establishing a background based on the first and the second magnetic images; capturing, using the array of sensors, a third magnetic image after a doped proppant is injected into a stage; and processing the third image to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage.
 2. The method of claim 1, wherein the array of sensor comprises magnetic sensors, wherein the magnetic sensors comprise diamond nitrogen-vacancy (DNV) sensors.
 3. The method of claim 1, further comprising arranging at least some sensors of the array of sensors on a surface area of the well pay zone.
 4. The method of claim 1, further comprising positioning at least some sensors of the array of sensors in locations within or near the well bore.
 5. The method of claim 1, wherein establishing the background comprises combining the first and the second magnetic images, wherein combining the first and the second magnetic images comprises subtracting the first magnetic image from the second magnetic image.
 6. The method of claim 1, wherein the proppant comprises a magnetic material, wherein the magnetic material comprises a ferromagnetic material, wherein the fluid comprises a fracking fluid.
 7. The method of claim 1, further comprising utilizing the information regarding distribution of the fluid and the proppant in the stage for adjusting fracking parameters before pursuing other stages, wherein fracking parameters comprise a fluid pressure and a time duration of application of the fluid pressure.
 8. The method of claim 1, further comprising beginning fracking from a first stage located at a toe of the well bore and continuing fracking other stages by moving away from the first stage.
 9. The method of claim 8, further comprising performing steps of the method after each new stage of the well to obtain updated information regarding distribution of the fluid and the proppant.
 10. The method of claim 9, further comprising processing images obtained after fracking multiple stages to obtain a distribution map of the fluid and the proppant, wherein the map comprises a three dimensional map.
 11. The method of claim 9, further comprising obtaining the distribution map of the fluid and the proppant by combining the information from magnetic images with information from other monitoring modalities, wherein the other modalities include micro-seismic mapping.
 12. A system for mapping and monitoring of hydraulic fracture, the system comprising: an array of sensors configured to capture magnetic images, wherein the magnetic images comprises a first magnetic image of a well pay zone, and a second magnetic image comprises a magnetic image captured after a well bore is padded with a fluid; and a processor configured to provide a background image based on the first and the second magnetic images, wherein: a third magnetic image is captured by the array of sensors after a doped proppant is injected into a stage, and the processor is configured to process the third magnetic image to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage.
 13. The system of claim 12, wherein the array of sensor comprises magnetic sensors, wherein the magnetic sensors comprise diamond nitrogen-vacancy (DNV) sensors.
 14. The system of claim 12, wherein at least some sensors of the array of sensors are arranged on a surface area of a well pay zone, wherein at least some sensors of the array of sensors are positioned in locations within or near the well bore.
 15. The system of claim 12, wherein the processor is configured to provide the background by combining the first and the second magnetic images, wherein combining the first and the second magnetic images comprises subtracting the first magnetic image from the second magnetic image.
 16. The system of claim 12, wherein the proppant comprises a magnetic material, wherein the magnetic material comprises a ferromagnetic material, wherein the fluid comprises a fracking fluid.
 17. The system of claim 12, wherein the processor is further configured to utilize the information regarding distribution of the fluid and the proppant in the stage to provide adjusted fracking parameters before pursuing other stages, wherein fracking parameters comprise a fluid pressure and a time duration of application of the fluid pressure.
 18. The system of claim 12, wherein the processor is further configured to process images obtained after fracking multiple stages to obtain a distribution map of the fluid and the proppant, wherein the map comprises a three dimensional map.
 19. A method comprising: capturing a first image of a well pay zone; capturing a second image after a well bore is padded with a fluid; providing a background image based on the first and the second images; capturing a third image after a doped proppant is injected into a stage; and processing the third image to subtract the background and to obtain information regarding distribution of the fluid and the proppant in the stage, wherein the first, second, and third images comprise magnetic images.
 20. The method of claim 19, wherein capturing the first, second, and third images are performed by using an array of magnetic sensors, wherein the magnetic sensors comprise diamond nitrogen-vacancy (DNV) sensors. 